The present invention relates to methods for purifying wet phosphoric acid using nanofiltration.
In wet phosphoric acid processing, phosphoric acid may be produced from phosphate rock (containing fluorapatite, or 3Ca3(PO4)2.CaF2). The phosphate rock may be attacked by concentrated sulfuric acid in a series of well-stirred reactors to produce phosphoric acid and calcium sulfate (gypsum) along with other insoluble impurities. The solids, primarily including gypsum, are separated off from the phosphoric acid. Fluoride may be removed at a further stage, e.g., as H2SiF6. Organic and inorganic solvents can be used to remove metallic and organic impurities. The purified phosphoric acid stream is then subjected to concentration by evaporation to produce commercial, merchant or food grade phosphoric acid.
The present inventors have recognized a need for improved methods for purification of wet phosphoric acid.